1. Field of the Invention
The present invention relates to methods and apparatus using a combination of hydraulic and electro-hydraulic control of a subsea blowout preventer (BOP) system.
2. Description of the Prior Art
Safety considerations in offshore drilling activities dictate that a subsea BOP must be able to rapidly close the well bore regardless of water depth at the drilling location. Conventional hydraulic BOP control systems experience unacceptable delays in operating subsea BOP functions in deep water applications because the time required to send a hydraulic activation signal through an umbilical hose from the surface control station to the subsea pilot control valve becomes excessively long in deep water. Additionally, delivery of sufficient quantities of pressurized operating fluid to the BOP function from the surface requires a substantial amount of time. These two elements of a complete BOP sequence time are usually referred to as signal time and fill-up time, respectively.
Existing methods for reducing signal time have included increased hose sizing and higher operating pressure, while fill-up time has been minimized through the use of subsea fluid storage accumulators to effectively reduce the distance some of the fluid must flow before reaching the BOP. The adequacy of these methods has been challenged by the desire to drill in waters more than 5,000 feet deep where conventional systems have drawbacks. Large diameter hose bundles in long lengths require substantial deck space for storage and pose running and retrieval handling difficulties. Also, the usable subsea accumulator volume diminishes with increasing water depth because of external hydrostatic pressure effects, thus forcing more accumulator bottles to be installed subsea as the water depth increases.
Although multiplex electric BOP control systems are known in the art, such systems are very expensive and complex. However, in order to drill in deeper water without experiencing reaction time problems, operators have found it necessary to replace existing hydraulic control systems with the more complex, more expensive multiplex electric BOP control systems. This is especially the case in ultra-deep water that is more than 5,280 feet deep.
Therefore, there remains a need for a BOP control system that can be used in deep waters without the slow communication of all-hydraulic systems or the complexity or unreliability of multiplex electric systems. It would be desirable if the BOP control system could be retrofitted to existing hydraulic control systems with minimal equipment modifications and installation onboard the drilling rig. It would be further desirable if the subsea portion of the control system were easily retrievable.
The present invention provides an apparatus for controlling a blowout preventer stack. The system includes a surface controller for transmitting hydraulic control signals and electronic control signals and one or more umbilical cables comprising a plurality of hydraulic control lines and a plurality of dedicated electronic control wires that extend from the controller to an umbilical junction plate. One or more retrievable control pod assemblies are provided with a pod junction plate that is selectively mateable to the umbilical junction plate The control pod comprises a plurality of direct operated solenoid valves in electronic communication with the controller through one or more of the dedicated electronic control wires. Each solenoid valve translates electronic control signals, such as application of 24 volts, from the controller into hydraulic control signals that are in communication with a hydraulically operated pilot valve to cause delivery of hydraulic fluid from a power fluid source to a critical function of the blowout preventer (i.e., closing of the blowout preventer). A suitable power fluid source includes, but is not limited to, an accumulator, an auxiliary hydraulic supply line, a dedicated hydraulic line in the umbilical, an auxiliary conduit on a riser, or combinations thereof
The system also provides a plurality of hydraulically operated pilot valves deliver hydraulic fluid from a power fluid source to a non-critical function of the blowout preventer upon receiving a hydraulic control signal directly from the controller through the umbilical. The system is preferably retrievable and does not include a multiplexer. It is preferred that the hydraulically operated control valves for critical functions do not receive a hydraulic control signal directly from the controller. The pod junction plate is selectively mateable with the umbilical junction plate under water, for example by a remote operated vehicle or a guide wire. Critical functions may be selected from, without limitation, the closing mode of one or more shear ram BOPs, the closing mode of one or more pipe ram BOPs and the closing mode of one or more annular type BOPs. Critical functions may include any other function considered essential in containing a kick or blowout from the well during drilling operations. The systems of the present invention are uniquely suited for operating in water of any depths, including water more than 5,000 feet deep, without requiring complex multiplexing technology.